Measuring a closed circuit voltage (CCV) and estimating an open circuit voltage (OCV) by use of the measured closed circuit voltage so as to estimate a state of charge (SOC) by use of the open circuit voltage is a known method for estimating a state of charge in a battery.
However, for a secondary battery which requires a long time until polarization is dissipated, it is difficult to estimate a state of charge accurately from an open circuit voltage because the battery has a large hysteresis upon charging/discharging in its SOC-OCV characteristics. A secondary battery in which, for example, SiO (silicon monoxide) is used for a negative electrode is a known secondary battery requiring a long time until polarization is dissipated.
Further, a remaining rechargeable-battery capacity detecting device which detects a remaining capacity in a rechargeable battery accurately is a known technology for estimating a state of charge. See, for example, Patent Document 1.
Patent Document 1: Japanese Laid-open Patent Publication No. 2001-281306